[sdiy] Single transistor Class A input/Output Impedance?

[Justin Owen]

Thanks Don,

>> On this standard textbook circuit - what sets the input and output impedance? Is it R2 or R1 || R2?

> (Heh-heh, the way you worded that is ironic.  A textbook circuit you say?)

:) Yes - 'textbook' as in that's the structure that appears most places - not as in 'just wire it up and it will work'. She's definitely more complex and demanding than she looks!

>The output impedance is, roughly, RL.
> The input impedance will be the parallel combination of R1, R2, and the resistive loading of the transistor's BE junction.

Thanks.

>> I've also seen versions (mainly for guitar pre-amp/boosters) with a Megohm R in parallel (in front of) with R2.

> Probably on the left side of the cap, and that would be standard practice to keep the cap from collecting a charge.

So in the schem I linked to, with e.g. a 1Meg R to GND in front of C1 - let's call it Rm - would that affect input Z?

Would the calculation then be  Rm || R1 || R2 ?

Thanks - appreciated.

- J


--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com
650 888-9632